The present invention is directed generally to a system and method for monitoring brakes of electronically controlled brake systems (“EBSs”) of vehicles and tractor and trailer vehicle combinations for overload.
A conventional EBS is designed to control brake lining wear and the adhesion relationship (that is, the coefficient of friction) between the vehicle wheels and the road surface. This is accomplished conventionally by controlling brake lining wear in the light braking range and adhesion in the heavy braking range.
When a vehicle driver depresses the brake pedal, the output brake actuation energy, in an initial range up to a presettable driver's set brake signal, is calculated solely according to criteria for controlling brake lining wear. In this initial range, axle loads are not considered. If the driver's set brake signal is reached, the control system switches from the light braking (brake lining wear) control range to the heavy braking (adhesion) control range. In the heavy braking range, comfort and safety aspects outweigh brake lining wear considerations.
The control objective in the heavy braking adhesion control range is equal adhesion—to achieve equal slip during braking for all vehicle wheels with no differential slip between vehicle axles. Axle loads are taken into consideration for calculation and control of brake actuation energy as the kinetic energy to be converted in the brakes is no longer equal from one axle to another. If set differential slip thresholds are reached before the driver's set brake signal is reached, the switch from the light braking brake lining wear control range to the heavy braking adhesion control range is triggered.
When, for example, a bus full of passengers is traveling downhill, the bus driver typically has to brake frequently, with the result that brake temperatures increase relatively quickly. As the brakes becomes hotter, the coefficient of friction (μ) of the friction pair of the brake (brake lining/brake disk or brake lining/brake drum) decreases, sometimes by as much as 100%. A decrease in the coefficient of friction of the friction pair translates into poorer braking power.
If the brakes of the vehicle's front and rear axles heat to different temperatures, as is usually the case, the reduction of braking power is greatest where the temperature is highest. Under such conditions, the differential slip deviates from normal. Such a differential slip is compensated for by the EBS—the control objective of which is a differential slip of zero.
In commercial motor vehicles, especially trucks or buses, the cooling conditions for rear axle brakes are typically poorer than those for front axle brakes. Moreover, rear axle loads are routinely about 50 to 100% greater than front axle loads. This means that the rear axle brakes become hotter than the front axle brakes—and this can occur even in the light braking brake lining wear control range.
Since the rear axle brakes are hotter than the front axle brakes, they generate less braking power at the same brake actuation energy. By virtue of the zero differential slip control objective of the EBS, more brake actuation energy is introduced into the rear axle brakes under EBS control. As a result, the rear axle brakes become even hotter, presenting a possible overheating situation and associated undesired reduction in braking power.
In tractor and trailer vehicle combinations (“vehicle trains”), it is known to superpose coupling force control on the electronic control of brake actuation energy in order to control the set pressure of the trailer vehicle train part. The objective of such coupling force control is a coupling force of zero, so that no force at all or only a force of permissible magnitude (set magnitude) is transmitted via the trailer coupling from the trailer vehicle train part to the tractor vehicle train part, or vice-versa, during braking. In this way, it is possible to prevent jackknifing of the vehicle train due to an undesired pushing effect by the trailer vehicle train part, and thus improve the directional stability of the vehicle train. Moreover, it is also possible to prevent an undesired pulling effect by the tractor vehicle train part during braking.
If the wheel brakes of the tractor and trailer vehicle train parts develop different wheel brake temperatures, e.g., due to different distribution of load between the tractor vehicle train part and the trailer vehicle train part, the braking power of the wheel brakes having higher temperatures will be less than the braking power of the wheel brakes having lower temperatures at the same brake actuation energy, and undesired pushing by the trailer vehicle train part or pulling by the tractor vehicle train part can develop. If the rear axle of the tractor vehicle train part is more heavily loaded, its wheel brakes become hotter than the wheel brakes of the trailer vehicle train part; and if the axles of the trailer vehicle train part are more heavily loaded, the wheel brakes of the trailer vehicle train part become hotter than the wheel brakes of the tractor vehicle train part. Hotter tractor vehicle train part brakes translates into pulling, and hotter trailer vehicle train part brakes translates into pushing. A possible overheating situation and undesired reduction in braking power can be presented if the brake temperatures reach a presettable, defined limit value.
EP 0 188 685 B1 describes a brake overload protection and/or overload warning device whereby an actual brake pressure is compared with a set brake pressure. If the actual brake pressure differs from the set brake pressure by a preset value, a warning signal is transmitted. This device relates to an individual brake.
EP 0 320 602 B1 describes an overload warning device for a trailer brake. Both the brake pressure in the tractor vehicle and the brake pressure in the trailer vehicle are measured. The ratio of the two pressures is formed in a comparator device. A warning signal is transmitted if the rate of change of the ratio becomes equal to or larger than a preset threshold value.
EP 0 417 431 B1 describes a method for monitoring a brake device for overload. The method relates to an individual drum brake. The deformation of a selected component (brake drum) is measured and compared with a stored deformation characteristic. If the actual deformation occurring during a braking process is equal to or larger than the stored deformation, a warning signal is transmitted.
EP 0 569 697 B1 describes a method for monitoring a trailer brake of a vehicle train for overload. The electronic brake system is equipped with continuous coupling force control. During braking, an energy demand signal is generated in the tractor vehicle. The generated energy demand signal contributes to determination of the energy supply to the tractor vehicle brake, and from this signal, a trailer energy signal is derived that contributes to actuation of the trailer vehicle brake, the coupling force developed in the coupling being controlled by means of the trailer energy signal. An overload condition is recognized and a warning signal is transmitted if the trailer energy signal exhibits, at a given energy demand signal, an increase that is not merely transient.
EP 0 569 698 B1 discloses a method for monitoring a tractor brake of a vehicle train for overload. As in EP 0 569 697 B1, the trailer energy signal is monitored, the only difference in this case being that an overload condition of the tractor vehicle brake is recognized and then a corresponding warning signal is transmitted if, as the result of coupling force control adjustments, the trailer energy signal exhibits, at a given energy demand signal, a decrease that is not merely transient.
In each of the foregoing known methods, physical variables such as pressure, force and torque are evaluated in a comparison process. If it is found during comparison that predefined limit values are exceeded, a warning is delivered to the vehicle driver, who must modify further driving actions appropriately to protect against further overload.
Accordingly, it is desired to provide a system and method for monitoring brakes of EBSs of vehicles and vehicle combinations for temperature induced overload, and for warning the vehicle driver of an overload condition.